1. Field of the Invention
The present invention is broadly concerned with improved rotary dryers which exhibit extremely high drying efficiency and are constructed so as to achieve a measure of impingement drying. More particularly, the invention is concerned with such dryers, as well as a gas turbulator design used therein and a method of drying, wherein the dryer includes first and second drying sections within a rotatable shell or drum, with each such section having a flow-diverting turbulator and a downstream serpentine flow section.
2. Description of the Prior Art
The drying of wood or agricultural particulates in a multi-stage dryer is dependent upon a large number of factors, e.g. the type of product to be dried, the initial moisture content thereof, particle geometry, variable ambient conditions, dryer configuration and fuels being employed.
In general, however, the drying process involves several distinct phases or stages. That is to say, most hygroscopic materials exhibit several distinct drying rate periods as they pass through a multi-pass dryer. Initial drying is accompanied by a warming of the material and its attendant moisture. The drying rate increases during this initial period, while the moisture content drops to a value which signals the beginning of a constant rate period of drying. During the constant rate period moisture is evaporated from the surface of product particles at a steady rate until the surfaces are no longer entirely wet. Thereafter, a falling-off period obtains where the drying rate decreases because of the increasing difficulty of moving internal product moisture to the particle surfaces where it can be taken up and moved away. Finally, the product moisture is reduced to a point where an equilibrium is established with the surrounding atmosphere.
Conventional three-pass dryers include an elongated horizontal, axially rotatable body having an outer drum and a series of concentric smaller diameter drums within the outer drum. The drums are in communication with each other and define a serpentine flow path within the dryer. Such dryers are provided with a product inlet oriented for directing initially wet product and hot drying air into the innermost, smallest diameter drum, whereupon the product is conveyed via induced draft current through the outer drum until it reaches a passageway defined by the outer drum and the next inboard drum. At this point the product is in its final fried condition and is delivered for further handling or collection. Thus, conventional three pass cylindrical dryers utilize comparatively high air velocities and temperature conditions in the innermost drum (first pass) where the incoming products are the heaviest and the wettest. Lower air velocities and lower temperatures obtain in the intermediate drum (second pass), and even lower velocities and temperatures exist in the outer drum (third pass). In practice, however, the relatively high air current velocity conditions in the first pass of a conventional dryer cause the wet product particles to be quickly driven away from the heat source, and there is consequently a reduced opportunity for adequate heat transfer and evaporation. In subsequent passes with lower air current velocities, the particles may settle out because the prevailing air current velocities fall below the saltation velocity of the product (i.e. the minimum air current velocity needed to pick up and convey product at a given moisture level). Thus, plugging of the dryer may occur, particularly at high product flow rates, and at best the product only moves at a rate determined by the forward velocity of the slowest moving (largest) particles. The result is that the flow rate is decreased and this inevitably has an adverse effect on drying efficiency.
U.S. Pat. No. 1,456,932 illustrates a dryer wherein vanes are located upstream of cylindrical drying areas along the length of the drum. None of these drying zones provide any reverse-flow serpentine arrangement. U.S. Pat. No. 3,571,944 provides an essentially conventional multiple-pass drum dryer but does not make use of any upstream flow diverting turbulator structure. Other references of general interest include: U.S. Pat. Nos. 5,285,581; 4,769,923; 4,633,595; 3,780,447; 2,470,315; 2,316,459; 4,802,288; and 4,945,657, and foreign patents DD 234,481; SU 1,196,638; SU 1,099,197; GB 1,581,542; and DE 1,812,954.
So-called impingement dryers have also been provided in the past. These dryers are characterized by design which directs incoming product to be dried into essentially direct intersection with a hot air drying stream. This type of dryer is very efficient, but heretofore the concept has not been usable in the context of rotary drum dryers.